for the most part, cost is a main determining factor in what you would be able to get:
X-Meridian is $199.95 USD at
http://www.sewelldirect.com
http://sewelldirect.com/AUZEN-X-MERIDIAN-71-sound-board.asp
(it wasnt at newegg that i saw)
X-FI is $92.99 USD at
http://www.newegg.com
http://www.newegg.com/Product/Product.asp?Item=N82E16829102188
X-Mystique is $69.89 USD at
http://www.amazon.com
http://www.amazon.com/Bluegear-Digital-Mystique-Gold-Windows/dp/B000BAV3SW
Turtle Beach Montego DDL (TBS-3300-01) is $49.99 USD at
http://www.amazon.com
http://www.amazon.com/TURTLE-BEACH-TBS-3300-01-Montego-Sound/dp/B0009MFQ4Y
beyond price, each card has their own individual benefits... the montego ddl and x-mystique are identical in regards to audio chipsets (both offer dd live 5.1, but the x-mystique includes an optical cable and has both optical and coaxial connections for digital, the montego ddl only has optical for its digital connections, and does not include an optical cable with it)... the successor to the x-mystique and montego ddl was the x-plosion (it offers dd live 5.1 and dts interactive 5.1)... the successor to the x-plosion was the x-meridian (dd live 5.1, dts interactive 5.1, high fidelity 7.1 analog)
as far as the x-fi, other people im sure can inform you well about it as far as features